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Appendix — Detailed Discussion on Development of TxID Assignments
Introduction
A Transmitter ID Code (TxID code) is a 13 bit number, which allows a total of 8,192 unique IDs. At the time of this writing, there are almost that many licensed facilities when coordinated international stations in the FCC's database are included. As more stations transition to ATSC 3.0 and distributed transmission systems, there will be a scarcity of TxID codes, requiring the reuse of codes that are separated geographically from each other to avoid interference between nearby stations on the same channel.
Three methods were applied to make efficient use of the available Transmitter IDs and to ensure that each licensed station has the largest reasonable number of TxID code values available to it.
The first method is to differentiate TxID codes based on channel number. Since a receiver/decoder will only identify co-channel TxID codes, facilities on different channels can share the same Transmitter ID without causing a conflict.
The second method is to assign different TxID code block sizes based on the sizes of broadcast footprint. Full service DTV and DTS facilities are assigned larger blocks of codes, and Class A TV stations, low-power TV stations, and TV translators are assigned smaller blocks of codes based on their smaller footprints. Note that Canadian and Mexican stations are assigned code blocks based on their coordinated facility types, just like US stations.
The final method is to assign codes based on geographic separation distance. It is imperative that two facilities on the same channel which utilize the same TxID code not be within reception distance of one another. To ensure this, facilities with a large broadcast footprint were studied. Based on these studies, the recommended minimum separation distance for full service facilities is 430 km. For Class A, low-power, and TV translator facilities, the minimum separation distance is 380 km.
Naturally, there is a tradeoff between how often codes can be reused, and the sizes of the blocks that can be assigned to each facility. Using the above guidelines, a block size of 128 codes can be used for the large full-service blocks, and a block size of 32 codes can be used for smaller, low power blocks. The TxID codes have been split into two groups: the large, 128-code blocks occupy the bottom half of the range of 8,192 IDs (0 – 4,095), and the smaller, 32-code blocks occupy the upper half of the range of ID's (4,096 – 8,191).
Methodology for assigning blocks
Since television decoders will only resolve TxID codes on co-channel stations, reusing a fresh set of numbers per channel is logical. Each channel is thus assigned TxID code blocks independently. The facility list is further subdivided by service, so that DTV and DTS facilities are in one group and Class A, DRT, low-power, and translator facilities are in another. When considering the facilities in a given channel group, the solution to assigning locally unique TxID code blocks has been borrowed from a "graph coloring" problem in the Graph Theory branch of mathematics. The goal is to optimize the distance between points without assigning the same color (or TxID code in our case). The software uses a heuristic called "Recursive Largest First" to assign a block of TxID codes to each facility with a specified minimum spacing. The end result is a table in which each facility has been assigned a TxID code block, and no two adjacent facilities have the same TxID code block if they are closer than the set distance limit.Software
In order to assign TxID code blocks to each facility using the methods described above, a software program was developed. The software outputs a CSV file (facilities.csv) as part of the assignment process, which is uploaded to the web page. The CSV file includes the following information, which is first obtained and filtered from the FCC's LMS database, then is updated by the program to provide First TxID and Last TxID, as needed:
| Column Name | Data Format |
|---|---|
| First TxID | Integer |
| Last TxID | Integer |
| Call Sign | Alpha Numeric |
| Facility ID | Integer |
| File No | Alpha Numeric |
| Service | Character |
| Channel | Integer |
| City | Character |
| State | Character (Abrev) |
| Country | Character (Abrev) |
| Latitude | Decimal Degrees |
| Longitude | Decimal Degrees |
Web page
The web page consists of an HTML file, including Javascript that contains the facility information that populates the main table on the web page. The downloadable CSV file, called facilities.csv, provides the same information for downloading in a human readable format.
At the top of the main table of the web page, there are options for filtering the table by Country, State, Call Sign, Facility ID, Channel, and assigned TxID code. There also are options for sorting the table by location, Call Sign, Channel, and TxID code. Multiple filtering options can be used at the same time. The sorting order of each column can be either ascending or descending. The call sign of each facility includes a link to FCCInfo.com, where more details of each referenced facility can be viewed.
As facilities change, either with channel changes or physical moves, new TxID code assignments may become necessary. When a new license is authorized by the FCC or its Canadian or Mexican equivalents, the new license will be compared to the older facility, and a determination is made whether or not to assign a new block of TxID code values.
Thoughts and Comments about the TxID Implementation
As noted in the ATSC 3.0 standard, the TxID code is primarily intended as a tool to aid adjustment of single-frequency networks (SFNs or Distributed Transmission System (DTS) networks) by enabling in-service measurement of relative signal levels and waveform arrival times from multiple network transmitters at receiving locations of interest. The TxID also can help in identifying interfering signals from nearby co-channel facilities and can be of use for this purpose regardless of whether the respective facilities involve single transmitters or transmitter networks. The relative power levels of desired and undesired signals can be determined, and solutions to interference can be developed to help optimize coverage.
So far, the largest ATSC SFN built had only eight licensed DTS transmitter sites. The current assignment process allows for significant growth in both the number of co-channel DTS networks, and the number of transmitters included in such networks. The TxID code value assignment method described is designed to permit allocation of additional blocks of codes to stations that exceed the numbers of values initially assigned, thereby supporting growth of networks through the addition of more blocks assigned to individual facilities, as needed.
While there are a number of ATSC 3.0 LPTV stations as this is written, there currently are no known LPTV facilities with more than one transmitter site, beyond possible experimental situations. Nevertheless, even single, low power facilities should be encouraged to include emission of TxID Codes in their ATSC 3.0 transmitters so that they will have in place the ability to identify and troubleshoot both environmental echoes and co-channel interference, thereby providing necessary tools for optimizing system service delivery.